1. Field of the Invention
The present invention relates to an apparatus and method for the reduction of printing offset in a bidirectional printing device, for example a dot-matrix printer.
2. Prior Art
Print heads for dot-matrix printers comprise printing elements such as needles, which are arranged in one or a plurality of rows. The individual needles or print elements are driven in accordance with information derived from a character generator, to result in printing a column of dots. Characters are formed out of successive dot columns. A column and print clock is supplied to the print head after the print head is moved by a column spacing, between successive columns for time purposes. The print clock defines the point in time in which the print elements are actuated.
Actuation of the printing elements always occurs during the movement of the print head at constant velocity. The formation of the print clock results by sensing movement of the carriage upon which the print head is mounted, or alternatively by sensing the drive means which moves the printer carriage. Specifically, the print clock signals are derived from scan clock signals which are typically produced by sensing movement of an optical timing disk or timing ruler. A so-called PLL module can be employed to derive the scan clocks from the timing disk or timing ruler, with the module producing an output clock signal of higher frequency in a predetermined phase relationship with the timing disk or timing ruler. The output clock signals are supplied to a print clock generator for the formation of the print clock signal.
In known apparatus this generation of the print clock signals enables columns of characters, and entire printing lines to be defined by means of counting the cycles of the print clock signal, beginning at the start of the line. The motion of the printer carriage must be decelerated at the end of a line and subsequently accelerated in the opposite direction. Since the print clock signals are only available during movement of the printer carriage, which occurs with instant velocity, the scan clock signal must be consulted during deceleration and acceleration, instead of the print clock signal. However this sometimes leads to errors in the form of a printing offset or mismatch, which can occur at the end of a printing event that is when switching from counting the printing clock cycles to counting the scan clock cycles. Such errors may be caused by mechanical characteristics such as stretching of the belt of a belt drive, or the flight time of the needles for dot-matrix printers with wire matrix print heads, or unavoidable tolerances of component parts.
As a result of these factors, it sometimes occurs that the last print clock signal lies immediately adjacent the edge of a scan clock signal, so that a time ambiguity of one scan clock cycle can occur. This then leads to the printing offset error mentioned above and to a poor print appearance. This result is particularly disadvantageous in connection with bidirectional printers, that is, printers which prints successive lines and two opposite directions.